1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method for forming an image based on the electrophotographic process.
2. Description of the Related Art
Conventional laser beam printers and copying machines which form an image by irradiating an image bearing member with a scanning laser beam are known as electrophotographic image forming apparatuses. Generally, such an electrophotographic image forming apparatus forms an image through a plurality of processes including the charging, exposure, development, transfer, fixing, and cleaning processes.
A general electrophotographic process will be described below. A charging unit uniformly charges a photosensitive member (image bearing member). An exposure unit exposes the charged photosensitive member to a laser beam according to an image signal to form an electrostatic latent image on the photosensitive member. The direction in which the laser beam scans is referred to as the main scanning direction. During rotation of the photosensitive member in the sub scanning direction perpendicularly intersecting with the main scanning direction, an electrostatic latent image is formed on the photosensitive member. Then, a development unit develops the electrostatic latent image on the photosensitive member to form a toner image on the photosensitive member. In toner image formation, the development unit charges toner and then applies the charged toner to the photosensitive member via a developing roller rotating at an approximately constant speed. The toner adheres to the electrostatic latent image to form a toner image. Then, the toner image on the photosensitive member is transferred onto a recording medium and is then fixed, thus completing image formation. Residual transfer toner on the photosensitive member is collected by a cleaning unit.
With such an image forming apparatus, horizontal streaks (hereinafter referred to as banding) by density variation may arise in the formed image because of various causes. There has been a problem that the image quality is remarkably degraded by banding.
For example, banding may occur when the rotational speed of the developing roller changes. When the developing roller rotates at high speed, the amount of toner supply increases to increase the amount of toner adhering to the electrostatic latent image, forming an image thicker than the intended image. When the developing roller rotates at low speed, the amount of toner supply decreases to decrease the amount of toner adhering to the electrostatic latent image, forming an image lighter than the intended image.
A method for correcting the amount of exposure based on the result of reading an image formed on an intermediate transfer belt is discussed in Japanese Patent Application Laid-Open No. 2007-140402.
If the image forming apparatus is continuously operated, a physical property of a device related to image formation changes, possibly changing the amplitude of the banding. However, the method discussed in Japanese Patent Application Laid-Open No. 2007-140402 performs, based on the result of reading a correction image at a certain timing, correction processing in a subsequent fixed time period. In some cases, therefore, this method may be unable to suitably correct the amount of exposure.